The present invention relates to a copper-clad steel trolley wire, composed of a core formed of a steel wire and a copper or copper-alloy covering layer thereon, and a method for manufacturing the same, and more particularly, to a copper-clad steel trolley wire enjoying high electrical conductivity, high strength, and improved wear resistance, and permitting easy detection of its wear limit, and a method for manufacturing the same.
Conventionally, copper wires or copper alloy wires are used for railroad-use trolley wires. The trolley wires have been required to have high electrical conductivity and improved wear resistance. The electric trains of nowadays are positively expected to travel at high speed, so that the trolley wires are required to have improved strength.
In order to increase the traveling speed of the electric trains, the wave propagation velocity of the trolley wires should be increased in advance. The wave propagation velocity C may be given by EQU C=.sqroot.T/d ,
where T is the wiring tension, and d is the linear density (weight per unit length) of a trolley wire.
As seen from the equation, the wave propagation velocity C can be increased by heightening the tension T or by lowering the linear density d.
Thereupon, an aluminum-composite trolley wire is proposed as a trolley wire with lower linear density d and higher propagation velocity C. This wire is composed of a steel wire and an aluminum covering layer pressure-bonded thereto.
Also proposed is a composite trolley wire which combines a copper-based material and an iron-based material of higher strength than copper. This trolley wire is improved in strength by the inclusion of the iron based material. By thus improving the strength of the trolley wire, the wiring tension T can be set higher than in the conventional case, to increase the propagation velocity C (see Japanese Patent Disclosure No. Sho 53-22786).
According to the aluminum-composite trolley wire, however, metal fixtures currently used are made of copper alloy, so that galvanic corrosion may be caused between the metal fixtures and the aluminum covering layer of the trolley wire. Such corrosion can be avoided only by forming the metal fixtures from a material which cannot suffer galvanic corrosion with aluminum. This countermeasure is not practical, however, in view of cost performance.
Although the copper-clad steel trolley wire is improved in strength and wear resistance, its copper covering rate is as low as 45 to 75%, so that its maximum electrical conductivity is lower than 80%. If the traveling section of an electric train is an AC section, the low conductivity arouses no problem. In the case of a DC section, however, the low conductivity results in substantial lowering of the current efficiency, therefore the trolley wire used should have high electrical conductivity.
As mentioned before, moreover, the trolley wire should be rewired when its residual diameter is reduced to a predetermined value corresponding to a wear limit. In order to detect the wear limit, the wire diameter must, for example, be manually measured at night throughout the wire arrangement, by means of an optical wire diameter measuring device. This measurement work, which is expected to be conducted periodically, is very troublesome. Thus, there is also a request for an improvement of the way of detecting the wear limit.
In the case of a composite trolley wire composed of iron-based material and copper-based material, furthermore, corrosion is liable to be caused at the boundary between the two materials, and there are many obstacles to practical use. A method for preventing corrosion is proposed such that a lead or tin layer is interposed between the iron- and copper-based materials. However, the interposition of such an intermediate layer is not advisable because it lowers the strength of the trolley wire and makes the manufacturing processes complicated.